1. Field Of The Invention
The present invention relates to so-called Faraday cups used for measuring various parameters associated with an electron beam, and more particularly is directed to a Faraday cup adopted for measuring the width, current density and energy density of an electron beam of generally strip-shaped cross-section.
2. Description Of The Prior Art
For controlling the generation of an electron beam by a gun having a cathode, anode and grid electrodes, and the focussing of the beam by electromagnetic or electrostatic focusing means, the voltage applied to one or more of the gun electrodes or the spacing therebetween, or the current or voltage supplied to the focusing means may be varied. In determining the voltage, current, and spacing variation that are required, various parameters associated with the electron beam, such as the width, current density and energy density are measured with the aid of a device, commonly referred to as a Faraday cup. For measuring these parameters, the electron beam is made to scan across an opening of the cup for producing a flow of current from the latter. More particularly, as the beam begins to cross the leading edge of the cup opening, current begins to flow from the cup and increases in value as more and more of the cross-sectional area of the beam is encompassed by the cup opening. Typically, the width of a conventional cup opening is larger than the cross-sectional width of the beam. Therefore, as more of the cross-sectional width of the beam crosses over the leading edge of the cup opening, more of the beam cross-sectional area is encompassed by the cup opening resulting in an increased current flow from the cup. Assuming that the value of the current reaches a maximum when the cross-sectional width of the beam has completed crossing the leading edge of the cup opening, the cross-sectional width of the beam can be determined from the scanning rate of the beam and the time required for the current to change from a minimum to a maximum value. Current density and energy density are readily determined from the maximum current value that is measured.
Electron beams can be characterized by their cross-sectional shape. For example, a beam of generally circular cross-section is commonly referred to as an electron spot beam whereas a beam of generally strip-shaped, cross-section is identified as an electron strip beam.
In the case of an electron spot beam of circular cross-section, the time required for the cross-sectional width of the spot beam to cross the leading edge of the cup opening is independent of the orientation angle between the scanning path of the beam and a normal to the leading edge of the cup opening. More particularly, the diameter of the spot beam is, in fact, the cross-sectional width of the beam measured at right angles to the leading edge of the cup regardless of such angle. Accordingly, the time required for the cup current to change from a minimum value to a maximum value is the same regardless of the orientation angle, and the measured beam width is not subject to variation and consequential error due to variation in the orientation angle. Furthermore, since the entire cross-sectional area of a typical spot beam fits within the cup opening, the maximum value of current measured with the cup, which is dependent on the maximum beam cross-sectional area encompassed by the cup opening, remains the same independent of the orientation angle. Therefore, the value of the orientation angle does not affect the current density or energy density of an electron spot beam determined by a conventional Faraday cup.
In the case of an electron strip beam, however, variation in the orientation angle results in variation in the beam width, current density and energy density as measured with a known Faraday cup. For example, if the length of the strip-shaped cross-section of the beam is parallel to the leading edge of the cup opening, that is, if the orientation angle is 0.degree. so that the strip beam approaches the leading adge of the cup opening in a direction parallel to the direction of the cross-sectional width, then the time required for the cup current to rise from a minimum value to a maximum value corresponds to the cross-sectional width of the beam and, of course, the scanning speed. However, when the orientation angle is such that the strip beam is skewed relative to the leading edge of the cup, the time required for the cup current to rise from a minimum value to a maximum current value will vary or be increased from the time required when the length of the strip-shaped cross-section of the beam is parallel to the leading edge of the cup opening. As can be readily appreciated, this increased time is determined by the orientation angle as well as by the cross-sectional width of the beam. Thus, the beam width cannot be accurately determined when the orientation angle is other than 0.degree.. Furthermore, the opening of a conventional Faraday cup cannot accommodate the entire cross-sectional area of the strip beam. Thus, depending on the orientation angle, different maximum cross-sectional areas of the strip beam will fit within the cup opening. Accordingly, the maximum current values flowing from the Faraday cup may vary with the orientation angle. Therefore, changes in the orientation angle can cause different and inaccurate current and energy densities to be measured with a conventional Faraday cup.
For avoiding such inaccurate current and energy density measurements, the size of the cup opening may be increased to encompass the entire cross-sectional area of the strip beam therein. However, in that case, the current distribution of the strip beam along the length of its strip-shaped cross-section cannot be measured. More particularly, a number of cups spaced along the length of the strip-shaped cross-section of the beam are necessary in order to determine the current distribution in the direction along the length of the strip-shaped cross-section.